Method and apparatus for controlling a heat exchanger

ABSTRACT

A method of controlling a heat exchanger such as a forced-flow boiler or the like is disclosed. The heat exchanger is supplied with the operating mediums of fuel and feedwater and generates slightly superheated steam. The method includes the steps of continuously determining the specific actual enthalpy of the slightly superheated steam and utilizing this enthalpy to control at least one of the operating mediums. The apparatus for performing the method of the invention includes sensing devices for sensing the pressure and temperature of the slightly superheated steam. A computation circuit of the apparatus determines the specific actual enthalpy as an electrical quantity to control an adjusting device for controlling at least one of the operating mediums.

United States Patent 1191 Borsi et al. 1 Nov. 27, 1973 METHOD ANDAPPARATUS FOR 3,244,898 5/1966 Hickox..l..' 122/448 x CONTROLLING A HEATEXCHANGER 3,284,615 11/1966 I I 3,338,054 8/1967 Hottenstme 60/107 [75]Inventors: Ladislaus Borsi, Karlsruhe; Helmuth' Dmglere worth both ofGermany Primary ExaminerAlan Cohan [73] Assignee: SiemensAktiengesellschaft, Assistant ExaminerAnen .ostranger h Mu i h Germany-NAttorneyl-lugh A. Chapin [22] Filed: Apr 14, 1972 I [57] ABSTRACT PP244,151 A method of controlling a heat exchanger such as a I force-flowboiler or the like is disclosed. The heat ex- [30] Foreign ApplicationPriority Data changer is supplied with the operating mediums of fuel A l4 1971 G P 21 18 028 9 and feedwater and generates slightly superheatedemany steam. The method includes the steps of continuously determiningthe specific actual enthalpy of the slightly [52] US. Cl 60/106,l22/;g2; l1222/g(5)/l2, superheated Steam d utilizing this enthalpy to[51] Int Cl F22; 35/18 trol at least one of the operating mediums. [58]Field of Search 60/105, 106, 107; The apparatus for Performing 1themethod the 122/448, 451; 236/11, 14 invention includes sensing devicesfor sensmg the 2 pressure and temperature of the slightly superheated 5References Cited steam. A computation circuit of the apparatus UNITEDSTATES PATENTS determines the specific actual enthalpy as an electricalquantity to control an adjusting device for controlling 2222222 2/22221:222:22 we 3,253,994 5/1966 Kagi 60/107 x 22 Claims, 8 Drawing FiguresPatented Nov. 27, 1973 b Sheets-Sheet 1 /%l l I Patented Nov. 27, 19733,774,396

5 Sheets-Sheet :3

Patented Nov. 27, 1973 5 Sheets-Sheet I5 Patented -Nov 1973 5Sheets-Sheet 4 ll|||l Patented Nov. 27,1973

5 Sheet-Sheet Fig.8

' BACKGROUND OF THE INVENTION The invention relates to a method andapparatus for Controlling a steam-generating heat exchanger such as aforced-flow boiler or thelike. More particularly, the invention relatesto a method and apparatus for con-' trolling at least one of theoperating mediums such as feedwater and fuel, supplied to the heatexchanger.

In a forced-flow boiler such as a Benson boiler, it is known to have anumber of primary control loops for controlling, for example, thefeedwater throughput, the output pressure of the steam delivered, thetemperature of the steam generated and the pressure in the combustionchamber. It has now been found advisable for the control of forced-flowboilers to provide, additionally, a control loop which serves for therapid detection and compensation of heating disturbances.

A supplementary control loop of this type is'known, in which the actualtemperature T, of the steam in the boiler is measured at a point wherethe steam is in a slightly superheated state. The term slightlysuperheated staterefers here and in the following to steam attemperature T in the range of 370C 40C, at a pressure p in the range of180 atm 100 atm and having a specific enthalpy i in the range of 640kcal/kg i 60kcal/kg.

In the known supplementary control loop, the measurement point for theactual temperature T, is at theof the energy carrier is noted mostquickly at the output of the evaporator, the energy carrier being, forexample, oil, gas or coal. The actual temperature T,- determined here iscompared with a predetermined reference temperature T,,, and thetemperature difference (AT T, -'T,,) obtained, is used as the correctionquantity for a subordinated control circuit, for example, forcontrolling the feedwater throughput. Because of this known measure, acertain degree of control of the slightly superheated steam has beenpreviously possible in ,a once-through forced-flow boiler when heatingdisturbances occur.

lthas now been found that a supplementary control loop configured in theforegoing manner has several shortcomings.

First, from an inspection of the enthalpy-temperature Example 1 7 Assumethat the slightly superheated steam is in a first state a defined by thetemperature T, 370C and the pressure p 200 atm. From the i-T diagram,the steam has an enthalpy of i, 610 kcal/kg. An increase of the enthalpyAi 30 kcal/kg (for example, because of a heating disturbance) resultsina temperature change AT 10C for a fixed pressure p A calculation showsthat this temperature change AT can be com'pensatedby a relative changeA M /M of the feedwater throughput'M of 8 percent. I

diagram (iT diagram) for water, it has been found that a'temperaturechange AT of the slightly superheated steamat the output of theevaporator caused, for exam ple, by a heating disturbance is notproportional to the heating disturbance. The change of the outputquantity Example 2 Assume that the steam now is in a second state bdefined by the temperature T,, 400C and the pressure p 200 atm. Thepressure has therefore not been changed relative to the first state a (pp,,). An enthalpy of i 680 kcal/kg is read from the i-T diagram. Ifthis'valueis-likewise increased by Ai, 30 kcal/kg,

a temperature change AT 27C results. In order to fully compensate forthis temperature change ,AT a relative change AM /M of the feedwaterthroughput of also 8 percent is required because Ai Ai,,". I

These two examples show that even if the pressureis kept constant (p,=p,, const).in the caseof an enthalpy change, a given relative change AM/M (for example 8 percent) of the feedwater throughput M results in adifferent temperature change AT depending upon the state a or b of theslightly superheated steam.

This relative change AM /M is brought about by a changed adjustment ofthe positioning valve. Furthermore, it follows from the i-T diagram thatthe temperaheating disturbances, are also picked up as being such aheating disturbance and are erroneously compensated for by a change ofthe adjustment of the positioning member, for example, for thefeedwaterthroughput. A temperature change AT in the slightly superheated steam ina heat exchanger can be caused not only by a heating disturbance; it canalso be caused by a pressure change Ap that is produced by a change inthe steam consumption. Another example will serve to illustrate this.

Example 3 The slightly superheated steam is assumed to be in the state cdefined by the temperature T 370C, pressure p 180 atm, and specificenthalpy i 650 kcal/kg. A pressure change of Ap 15 atm, which may becaused, for example, by a change in steam withdrawal, leads with nochange in the heating, that is, with constant enthalpy i to atemperature change AT 10C which, although not caused by a heatingdisturbance, automatically results in an unnecessary control action inthe known control loop.

Pressure changes of this kind occur very frequently, especially when thesteam generator is operated at varying pressure, that is, if thepressure p, changes with a change in load, or if its storage capacity isheavily drawn upon or more specifically, if the steam output deliveredis predetermined over a large pressure range, for example, by asuperordinated control of the electric power output of a connectedturbogenerator.

SUMMARY OF THE INVENTION It is an object of the invention to provide animproved method of control for a heat exchanger such as a once-through,forced-flow boiler or the like.'Subsidiary to this object it is anobject of the invention to provide such a method for detectingandcompensating for an operational disturbance such as a heatingdisturbance. V l

It is another object of the invention to provide a control method for aheat exchanger for controlling at least one of the operating mediums offuel and feedwater supplied to the heat exchanger in response to aheating disturbance. I I

It is still another object of the invention to provide an apparatus forcarrying out the above method objects.

According to the invention, the foregoing method objects are realizedbycontinuously determining the specific actual'enthalpy (i of theslightly superheated steam in the heat exchanger and using this quantityas an auxiliary control quantity.

A preferred embodiment of the invention includes continuously measuringthe pressure (p,) and temperature (T,) of the slightly superheated steamat a location of the heat exchanger. The measured actual values ofpressure (P,) and temperature (T,) are supplied to a computation circuitfor determining .the control deviation (x,,,,) of the specific actualenthalpy (i from the reference value of the enthalpy (i and the controldeviation (x 0f the specific enthalpy is used as a correction quantityfor the input quantity of an adjusting means. The adjusting means caninclude a controller to which the inpu'tquantity is supplied. Theadjusting means further can include positioning equipment connected tothe controller. The positioning equipment can, for example, in turninclude a valve for adjusting the supply of feedwater to' the'heatexchanger as well as a positioning device connected to the valve foradjusting the position of the valve. It is also possible to of fuel tothe heat exchanger.

If the positioning actions of a positioning device are derived in themanner indicated from the control deviation x,,,, of the specificenthalpy, the relative loop gain is constant at any operating point Twhich is considered as a special advantage. If, for instance, afeedwater positioning valve is used as the positioning member, thequantity (Ai/( AM /M is constant for any temperature T Expressedotherwise, an enthalpy change Ai is directly proportional to the changeAM /AM of the feedwater flow. In contrast to the known control method,in which only the actual temperature T, of the slightly superheatedsteam is measured, there occurs therefore no nonlinearity in the gain ofthe temperature control loop (operating point and drive dependence).

In a further embodiment of the method according to the invention, thecontrol deviation x of the specific enthalpy is determined from themeasured actual values p, and T, and from predetermined reference valuesfor the pressure p,, and for the temperature T,,.

The specific enthalpy i is the determiningfactor for the equilibriumbetween the heat supply and the feedwater flow and is an unambiguousfunction of the temperature T and the pressure p:

For the deviation (i i from the desired reference state, which is to bedefined by the quantities i T and p the following approximation isobtained:

With the definitions customary in control engineering: x,,,, i, irepresenting the control deviation of the specific enthalpy, x p,- p,,the control deviation of the pressure, and x, T, T the control deviationof the temperature, the following equation is obtained:

x 8i/6p) x (8i/8T),, x

The quantities and T denoted with the subscript i are the actual values.'For small changes about a fixed operating point (i p,,, T the partialdifferential quotients contained in equation (4) can be considered asprovide a similar arrangement for adjusting the supply constants. If onethus sets I( (Si/8p and K ('8i/6T),, the defining equation for thecontrol deviation of 'ihe specific enthalpy is obtained as:

I wi v wv r w'l I i i Accordingly, a further embodiment of themethod forcontrol on a heat exchanger according to the invention is thereforegiven by providing that the control deviation X,,,, of the specificenthalpy is calculated according to equation (5) above, wherein X, andK, are predetermined values and wherein x,,,,, denotes the controldeviation (p p of the pressure and x, the control deviation (T,- T )'ofthe temperature.

If the control deviation x is to be determined for an extended operatingrange, the parameters K,, and K can no longer be considered asconstants. A further embodiment of the method according to the inventionconsequently provides for this case and the values K and K arecontrolled as functions of the load, particularly in dependence on thereference value P, of the electric power of a turbogenerator connectedto the heat exchanger. correspondingly, the reference value p of thepressure and the reference value T, of the temperature can also becontrolled. In this manner, control in the varying pressure mode canalso be realized. In the special case that all four values K K p and Tare simultaneously controlled as functions of the load, for

example, of the reference value P of the electric power of theturbogenerator, equation assumes the form;

The enthalpy deviation x,,,, can therefore be determined for thisspecial case according toequation (6).

ln the following, further possibilities for the continuous determinationof a small control deviation x,,,,- from the operating point A(reference state i,,, p T of the.

slightly superheated steam willbe illustrated. Again,

the starting point isthe total differential quotient, equation (2), ofthe specific enthalpy 1. Since the following applies as general:

The expression in the bracket on the right-hand side in 6 operatingpoint p,,, i,, a positive temperature change AT,, +9C is, on the otherhand, equivalent to an enthalpy change of the same absolute magnitude ofAi kcal/kg ='Ai,," -,that is, one obtains in this instance for thequantity: (Ai AT,',' 2.22

For a constant reference enthalpy i,,, the quantity (Ai/ AT),, ispractically only a' function of -'x',,.-, and essen- I diagram revealsthat the isobaric lines are, in good apequation (8) is apressure-dependent quantity, which I can be considered as apressure-dependent auxiliary or reference temperature T,,:-

s s o T a 1J (pi p0) with one obtains for the control deviation x,,,; ofthe specific enthalpy:

= (A i/ A T),; x'

' (12) The quantity ('Ai/ AT) therein cannot be assumed as constantfitdepends on the magnitude and the sign of x T, T,.(p,). This will beillustrated by an example:

Example 4 i Let the slightly superheated steam be in a state d definedby the pressure p,, 220 atm and by the specific enthalpy i 630 kcal/kg.When the steam is subjected 1 to a negative temperature change of A T,,-7C with the pressure held constant, it is accompanied by an enthalpychange A i -20 kcal/ kg. in this way, the

quantity (Aid/AT),;,,= 2.86 kcal/C kg For the satne proximation,parallel for fixed referenceenthalpy i,,.

Starting from equation 12), there are,,according to the invention','twofurther embodimentsto determine the controldeviation x of the specificenthalpy and to use this quantity for control purposes:

Thefir'st of these embodiments comprises the moregeneral case and isprovided for :the sliding pressure mode of operation (p, variable) ofthe heat exchanger and for constant reference enthalpy i,, of theslightly superheated steam. According to a'feature of this 'embodiment,an auxiliary quantity, designated at a pres-' sure-dependent referencetemperature T is formed as a function of the measured actual pressurep,-. Also, a temperature deviation (T, T is determined by subtractingthis reference temperature T,- from the measured actual temperature T,-,and the control deviation x of the specific enthalpy is determined as afunction of the temperature deviation (T,- T In a circuit arrangementfor carrying out this method embodiment, each of the quantities T, and Ai/ A T),, are most advantageously formed in respective functiongenerators.

The second embodiment is suited particularly for fix ed-pressureoperation (p,, approximately constant) of the heat exchanger and forconstant reference enthalpy i of the slightly superheated steam.According to a featu're of this embodiment, the control deviation x ofthe specific enthalpy is calculated according to equa- 6. .9.1 2).

wherein the quantity Ai/A T),,' is predetermined as a quantity dependenton the operating point (i,,, p,,, T,,) of the slightly superheatedsteam, and the pressuredependent reference temperature T is determinedaccording to equation (9) as:

I .t= o T m (pi fpu) j with fixed a In acircuit arrangement with imple-0 0 mentmg this method, the dependenceof the quantity A|'/ A 'I),, istaken into consideration by a function generator. The pressure-dependentauxiliary quantity T, on the other hand, can be determined according toequation (9) in a simple computing circuit.

There is also the further embodiment in which the reference enthalpy iof the slightly superheated steam is assumed not to be constant.instead, the reference enthalpy i is assumed as fluctuating about a meanrefer'ence value T A change Ai i i from the mean reference enthalpy ican be brought about intentionally in the operation of a heat exchanger.Such an intentional change Ai can occur, for example, through control,if thereference enthalpy i,, is controlled in dependence on the load;however, it can also be caused by the action of a superordinated controlif, for example, after a soot blowing operation, the distribution of theheat flow over the individual heating surfaces of the heat exchanger haschanged.

A still further embodiment of the invention for the case where the heatexchanger is operated with a reference enthalpy i,, which varies about amean reference value T In this embodiment, a pressure dependentreference temperature T is formed as a function of the measured actualpressure p,- and a temperature deviation (T T is determined bysubtracting the pressuredependent reference temperature T, from themeasured actual temperature T,-. Also, an enthalpy deviation x5, isdetermined as a function of this temperature deviation (T, T,,), and,for the purpose offorming an effective deviation 'x for use as anauxiliary control quantity, the'intended enthalpy deviation Ai i i issubtracted from the enthalpy .deviation x determined in the abovemanner. The intended en'- thalpy change Ai can here be controlled as afunction of the'load, for example, 'as'afunction of thereference value Pof the electric power of a turbogenerator connected' to the heatexchanger.

Although the invention is illustrated and described herein as a methodand apparatus for controlling a heat exchanger, it is nevertheless notintended to be limited to the details shown, since various modificationsmay be made therein within the scope and the range of the claims. Theinvention, however, together with additional objects and advantages willbe best understood from'the following description and in connection withthe accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the method andapparatus are illustrated in the following eight FIGS. described below.Similar or likecomponents are designated by the same reference numeralin each FIG. in which they appear.

FIG. 1 is a signal flow diagram for controlling the feedwater in aBenson boiler according to the method and apparatus of the invention.

FIG. 2 illustrates a computing circuitfor determining the controldeviation x 'of the specific enthalpy for a fixed operating point P T ofthe slightly superheatedsteam FIG. 3 is a computing circuit fordetermining the con- I trol deviation x,,,, for constant referenceenthalpy i and fixed pressure p for fixed-pressure operation.

FIG. 7 illustrates a'computing circuit for determining the effectivecontrol deviation x,,,, in view of an intended change AL, of the meanreference enthalpy t FIG. 8 is a signal flow diagram for the feedwatercontrol in a Benson boiler which includes superimposing the effectivespecific enthalpy deviation obtained according to FIG. 7.

- is designated by reference numeral 7 and is determined DESCRIPTION OFTHE PREFERRED EMBODIMENTS Referring now to FIG. 1, a Benson boiler l issupplied with thermal energy bymeans of an energy carrier 2 forevaporating the inflowing feedwater 3. The energy carrier, for example,can be oil, gas or coal. After passing througha feedwater valve 4 thatis connected to a positioning drive 5 controlled by an electric motor,an injection water stream 6 is diverted away from the feedwater 3. Thefeedwater flow less the injection water 6 Ma flow meter 8, and convertedinto an electrically measuredquantity M by a measuring transducer 9. Theflow meter 8 can be in the form of a measuring orifice. After passingthrough the flow meter 8, the feedwater throughput 7 is preheated in'apreheater 10 of the Benson-boiler 1 and is vaporized in an evaporation11; The steam leaving the evaporator 11 is slightly superheated. .It isthen conducted to a first steam superheater 12. From there thesteam isfed to an injection cooler 13, into which the diverted injection water 6is fed through an injection water valve. l4.' The steam then flows to a.further steam superheater 15. The steam 16 leaving the Benson boiler lis subsequently fed to a turbogenerator (not shown).

A temperature transmitter 17 is placed between the evaporator 11 and thefirst steam superheater 12 for continuously measuring the actual valueof the temperature of the slightly superheated steam. A temperaturetransducer 18 connected to the transmitter 17 converts this value intoan electrical quantity T,-. At the same location, there is placed apressure transmitter 19 for continuously measuring-the actual pressurevalue. A pressure transducer 20 is connected to the pressure transmitter19 that converts the pressure into a corresponding electrical quantity pelectrical output quantity x using these quantities.

The quantity x is a measure of the deviation of the specific actualenthalpy i,- fromthe specific reference enthalpy i f ln the basic flowdiagram shown in FIG. 1, the formation of. the output quantity x takesplace with the aid oftwo fixed, predetermined electrical quantities Tand p,,, which are generated in respective reference value transmitters22 and 23 according to the desired temperature and pressure of theslightly superheated steam.

Embodiments of the computing circuit 21 which are adapted to a givenrequirement are described below with reference to the following FIGS.

According to FIG. 1, the output quantity x,,,, of the I set as a fixedvalue by a reference setting device (not shown) or is controlled asaifunction of the load. The input quantity (2 of the regulator 25 is ameasure of the deviation of the measured throughput from the adjustedreference throughput. The positioning quantity y delivered by theregulator 25 acts on the feedwater equation (5 valve 4 in a directionminimizing the deviation "of the feedwater throughput from its referencevalue.

Should a heating disturbance as, for'example, a temporary change in thecalorific power of the energy carrier 2 occur; this is noted veryquickly as a change of the specific enthalpy i, of the slightlysuperheated steam at the output of the evaporator 11, and the outputquantity x,,,, of the computing circuit 21 fed to the comparator 24causes, via the regulator 25, a change in the feedwater throughput in adirection to keep the specific enthalpy constant i,-. In the embodimentaccording to FIG. 1 the output qu'antity'x of the computing circuit 21thus serves, as a correction quantity for the input quantity e of theregulator 25.

FIG. 2 illustrates a computing circuit designated 21a which can be usedwith a heat exchanger or oncethrough, forced-flow boiler in which theoperating point of the slightly superheated steam is determined bypressure and temperature and is essentially fixed, that is, notcontrolled. In order to simplify the description, the electricalquantities of current and voltage oc-' curring in the electricalcircuits which are proportional or. correspond to the physicallymeasured values of pressure, temperature or quantities of specificenthalpy are designated here and with reference to the remaining FIGS.asthe physically measured values andquantities to which they refer.

The control deviationx of thespecific enthalpy is determined in thecomputing circuit 21a of FIG. 2 by The values K, and K are hereconstants adapted to the operating point. The preset reference value pof the pressure is subtracted from the continuously measured actualvalue p of the pressure at the subtraction circuit 26. The difference xp, p is multiplied by the constant K, at the multiplier 27. The productK x is fed to the first input of an adder 28. At the same time, thereference value T is subtracted from the continuously measured actualvalue T, of the temperature at the subtraction circuit 29; and thedifference x, T,' T is multiplied by the constant K, at anothermultiplier 30. The product K Jr, is fed to the other input of the Yadder 28. At the output of the adder, and therefore at the output of thecomputing circuit 21, there is provided thecontrol deviation x of thespecific enthalpy, determined according to equation Example 4 below willillustrate howthe constant K and K are determined for a fixedoperatingpoint.

Example 4 The operating point A of the slightly superheated steam willbe assumed as given by the temperature T,.

= 370C, by the pressure p 180 atm and so, by the 10 thesevalues inequation (5 one obtains as the defining equation for x,,.,-: I

x,,, Ikcal/kg] 1.45 k [atm] 2.18 x, [deg].

FIG. 3 shows a computing circuit designated 21b for determining thecontrol deviation x in which the pressure reference value and thetemperature reference value T are no longer fixed, preset constants ofthe slightly superheated steam; they are rather controlled viarespective function generators 31 and32 as functions of the electricpower P, which'a turbogenerator connected to the heat exchanger is todeliver.

,In FIG. 4 is shown a computing circuit designated 21c. with thiscircuit, the enthalpy deviation x (P according to equation (6) givenabove is determined. In contrast to the computing circuit of FIG. 3, thevalues K, and K are now also controlled as functions of the desiredelectric power P, via respective function generators 33 and 34. Thecomputing circuit is of particular importance for the case that thecontrol deviation x is to be determined for anextended range of.operation, where the values of K,, and K can no longerv be considered asconstant. I.

FIG. 5 shows acomputing circuit designated 21d, in which the controldeviation x,,,,- of the specificenthalpy is determined on the basis ofequation (12) for varying pressure operation 1,, variable) and constantreference enthalpy i,, of the heat exchanger. A reference temperai tureT which is a function of the actual pressure p is determined as anauxiliary quantity withthe aid of the function generator 35. Thispressure-dependent reference temperature T, is subsequently subtractedin a subtraction circuit 36 from the measured actual temfers from theforegoing embodiments only with respect to the realization of thepressure-dependent reference temperature T,,.. It is applicableforfixed-p'ress'ure oper ation (p const), specifically, if the heatcontent of the slightly superheated steam is to be kept constant,

for example, the steam at the output of the evaporator 11 in FIG.l.-Thepressure is switched in here linearly without the use of thefunction generator 35 shown in FIG. 5. For this purpose, the measuredactual pressure p, is first multiplied by a constant a o inalmultiplier38. In an adder 39, the summation a1... .,p1-+T aT,,, p0 according toequation (9) is subsequently made. The sum formed in this manner isequal to the pressure-dependent reference temperature T, and is fed tothe subtraction circuit 36 as shown in FIG. 5.

i There are applications, in which'the reference enthalpy i of theslightly superheated steam cannot be assumed as constant, but isintentionally changed, for example, by superordinated controls. For suchcases the circuit configuration shown in FIG. 7 and designated 21f isprovided. An enthalpy deviation designated by the symbolf, is determinedin the computing circuit 21f, which corresponds to the computing circuit21d shown in FIG. 5. The control deviation x, is effective as anauxiliary control quantity and is generated in a subtraction circuit 40by subtracting an intended en-' thalpy change Ai, from the enthalpydeviation x determined by electronic circuits from the values of p, andT1.

The intended enthalpy change AL, is formed inturn in an adder 41 from afirst'enthalpy value Ai, and a second enthalpy value Ai The firstenthalpy value Ai, is delivered tothe adder 41 by a control 42 accordingto an enthalpy deviation x which is' formed in the circuit 44 fromvalues of P,- and T measured, for example, at the output of the steamsuperheater 12 following the evaporator 11 of FIG. 1. The secondenthalpy quantity Ai," is determined, for example, by a functiongenerator 43 as a function of the desired value of the power P, of theturbogenerator connected to the heat exchanger. Y

FIG. 8 shows a control circuit in a heat exchanger; this configurationconstitutes a variant of the usual feedwater controlsystem in a Bensonboiler 1 and uses the specific enthalpy for control purposes. The flowdiagram of the control circuit shown in principle in FIG.

' l is supplemented and modified essentially by the circuitconfiguration shown in FIG. 7. Referring to FIG. 8, an enthalpy quantityAL, is provided at the output of the Pl (proportional-integrating)control 42. The enthalpy quantity Ai, is formed with the Pl control 42from the deviation x',,, A T of the temperature difference (T T,,) fromits reference value, the temperature pertaining to the intenselysuperheated steam and being measured at the injection cooler 13.

The effective enthalpy deviation x determined in the circuitconfiguration of FIG. 7 is linked at a multiplier 44 with a referencevalue P, to the reference value M W of the feedwater throughput. Thevalue P is equal, for example, to the desired value of the power outputof the turbogenerator following the Benson boiler 1. The measuredfeedwater flow M is compared in a subtraction circuit 45 with thisreference value M and the difference (M M formed there is fed to a PIcontrol 46. The positioning drive of the feedwater valve 4 is controlledin accordance with the positioning quantity y delivered by the Plcontrol 46.

What is claimed is:

1. Method of controlling a heat exchanger such as a forced-flow boileror the like wherein slightly superheated steam is generated, comprisingcontinuously determining the specific actual enthalpy (i,-) of theslightly superheated steam, and utilizing the specific actual en-.thalpy as an auxiliary control quantity.

2. The method of claim 1 wherein the heat exchanger is supplied with theoperating mediums of fuel and feedwater and is equipped with adjustingmeans for adjusting the flow of at least one of the operating mediums tothe heat exchanger, comprising continuously measuring the actualpressure (p,) and actual temperature (T,-) of the slightly superheatedsteam in the heat exchanger, supplying the measured actual values ofpressure (p,) and temperature (T to a computation circuit fordetermining the control deviation (x,,,,) of the specific actualenthalpy (i,) from the reference value of the enthalpy (i and supplyingthe control deviation (x,,,,) of the specific enthalpy as a correctionquantity for the input quantity of the adjusting-means. 3. The method ofclaim 2: wherein the adjusting means includes a controller connected toa positioning device for adjusting the flow of one of the operatingmediums to the heat exchanger, comprising supplying the controldeviation (x,,,,) of the specific enthalpy as a correction quantity forthe input quantity (e) of the controller.

suppliesa load and wherein the values of pressure p,,)

4. The method of claim 2 comprising supplying respective predeterminedreference values of pressure (p,,) and temperature (T,,) for theslightly superheated steam to the computation circuit, and determiningthe control deviation (x,,,,-) of the specific enthalpy from themeasured actual values of the pressure (p,-) and temperature (T,-) andthe predetermined refemce values of pressure (p,,) and temperature (T i5. The method of claim 4 comprising determining the specific enthalpy(.xwi) according to the equation:

wherein K p and K are predetermined values and wherein (x is the controldeviation (p p of the pressure and x is the control deviation (T,- T ofthe temperature. 1 I t 6. The method of claim'5 wherein the heatexchanger and temperature (T are functions of the load.

7. The method of claim 5 wherein the heat exchanger supplies a load andwherein the values of (K,,) and (K of electric capacity, and wherein thevalues of pressure .1

(p and temperature (T are functions of (P 10. The method of claim 5wherein the heat exchanger is connectable to a turbogenerator forsupplying the same, the turbogenerator having a reference value (P ofelectric capacity, and wherein the values of (K,,) and (K are functionsof (P,,). I

11. The method of claim 5 wherein the heat ex.-

changer is connectable to a turbogenerator for supplying the same, theturbogenerator havinga reference value (P of electric capacity, andwherein the values of pressure (p0) and temperature(T,,) and values of(K,-,) and (K,) are all functions of (P,,).

12. The method of claim 2 wherein the heat ex- 5 changer operates underthe condition that the reference value of the pressure (p,,) is variableand the reference value of the enthalpy (i of the slightly superheatedsteam is constant, and wherein the method comprises forming a pressuredependent auxiliary quantity as a function of the measured actualpressure (p,-), said quantity being in the form of reference temperature(T subtracting the reference temperature (71,) from the measured actualtemperature (T,-) to determine a temperature deviation (T 1.), anddetermining the control deviation (x,,,,-) of the specific enthalpy as afunction of the temperature deviation (T,- T

' 13. The method of claim 2 wherein the heat ex- 7 changer operatesunder the condition that the reference value of the pressure {p,,) issubstantially constant and that the reference enthalpy (i,,) of theslightly superheated steam is constant, and wherein the method comprisesdetermining the control deviation (x,,,,-) of the specific enthalpyaccording to the equation:

x,,. (minn w wherein the quantity (Ai/AT), is predetermined as dependentupon the operating point (i,,, p, T,,) of the slightly superheatedsteam, and wherein the quantity (T,,) is a reference temperaturedependent upon pressure and determined from the equation:

pendent reference temperature (T as a function of the. measured actualpressure (p,-), subtracting the refer-.

ence temperature (T,,) from the measured actual temperature (7}) todetermine the temperature deviation (T, T determining a controldeviation quantity (f asa function of the temperature deviation (T,7),), and subtracting an intended enthalpy change (Ai i, i,,) from thequantity (f to formthe control deviation (x,,,,).

115. The method claim 14 wherein heat exchanger supplies a load andwherein the intended enthalpy change "(M is a function of the load.

16. The method of claim 15 wherein the heat exchanger is connectable toa turbogenerator for supplying the" same, the turbogenerator having aspecified value (P of electric capacity, and wherein the intendedenthalpy change (Ai is a function of (P 17. The method of claim 14wherein the heat exchanger is equipped with an injection cooler suppliedwith intensely heated steam and a superordinated controller, comprisingdetecting the temperature difference (T', T',,) of the intenselysuperheated steam at the injection cooler and forming therewith thedeviation quantity (x' A and supplying the deviation quantity (x',,, Ato the superordinated controller to form the quantity (Ai' forcorrecting the enthalpy deviation (Ai 18. In a heat exchanger such as aforced-flow boiler or the like supplied with the operating mediums offuel and feedwater and having an evaporator wherein slightly superheatedsteam is generated, the heat exchanger being equipped with an apparatusfor controlling the heat exchanger, the apparatus comprising pressuredetection means and temperature detection means for continuouslymeasuring the pressure and temperature respectively of the slightlysuperheated steam at the output of the evaporator, computation meansconnected to said detection means for continuously determining thespecific actual enthalpy of the slightly superheated steam, and meansconnected to said computation means for controlling the heat exchangerin response to said specific actual enthalpy.

19. The apparatus of claim 18 wherein the apparatus controls at leastone of the operating mediums said lastmentioned means being adjustmentmeans for adjusting the flow of at least one of said mediums to the heatexchanger in response to saidspecific actual enthalpy.

20. The apparatus of claim 19 wherein the heat exchanger is equippedwith a conduit to supply the feedwater tothe heat exchanger proper, saidadjustment means comprising positioning equipment, said equipmentincluding a valve for controlling the supply of feedwater to the heatexchanger, and a positioning device connected to said valve foradjusting the position i of said valve,

21. The apparatus of claim 19 wherein the heat ex- 1 changer is equippedwith a conduit to supply the fuel to the heat exchanger proper, saidadjustment means comprising positioning equipment said equipmentineluding a valve for controlling the supply of fuel to the connected tosaid pressure transmitter for converting said pressure to acorresponding electrical quantity; said temperature detection meanscomprising a temperature transmitter placed at the output of theevaporator, and a temperature transducer connected to said temperaturetransmitter for converting said temperature to a correspondingelectrical quantity; said computation means comprising respectivereference transmitters for providing respective predetermined electricalreference quantities of pressure and temperature for the slightlysuperheated steam, and further computation circuit means connected tosaid reference transmitters and said transducers for forming a controldeviation signal corresponding to the deviation of the specific actualenthalpy from the reference value of the enthalpy; and said adjustmentmeans comprising means responsive to the control deviation signal foradjusting the flow of at least one of the operating mediums to the heatexchanger.

STATES PATENT OFFICE fICERTIFICATE OF CORRECTION -"Patent so. 77#,396 fDateel Nm rember 27; 1973 I eteifhgrento'fls) Iacfiij'slaius B 1 51;fneimuth Dingle! It is ce'tti fid that ierrbr eppearein theaboveidentified patent end that said Letters Patent are herebycorrect-ed as shown below:

nw l m 7,1irie- 12,1charige "1%". to -'-xa$i I tamed thrpughoqt text g IIII"! colilnl l'i 1 l 55ibhd A 0 0 t) to column; f line [63 c hajnge"mean reference enthalpy 1 to e riir f enthalpy o" t "column 9,1 1i ne63, change 19 (Ai/AT) kc al/kg/deg" i QYFfKT -F /Amp E e ee-w tin-columnlEQH-Iihe, change "of reference" to --of arefefence Sigti'ed,efidfisealed this 17th day of September 1974.

v (SEAL) E Attest: v e e WMecoy IBSON J c. MARSHALL DANN I K "Attestirxg Qf fi ce r 1 r Comiss ioner of Patents FORM Po-foso so-s9) UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,77U- ,396Dated November 27, 1973 Inventor) Ladislaus Borsi, Helmuth Dingler It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In column 7, line 12, change "Xfii" to --X$j and throughout text.

In column 7, line 15, change "A1 i 1 to --Ai i0 IT In column 7, line63,change "mean reference enthalpy 5. to

mean reference enthalpy 1 In column 9, line 63, change "K /(Ai/AT) 2.18kcal/kg/deg" 0 ----'K:I\ (Ai/AT) 2.18 kcal/kg/deg-- In column 12, line51, change "of reference" to --of a reference Signed and sealed this17th day of September 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C.' MARSHALL DANN Attesting Officer Commissioner ofPatents FORM P0-1050 (10-69) USCOMM-DC 60376-P69 i UIS. GOVERNMENTPRINTING OFFICE IBIS 0-366-331,

1. Method of controlling a heat exchanger such as a forced-flow boileror the like wherein slightly superheated steam is generated, comprisingcontinuously determining the specific actual enthalpy (ii) of theslightly superheated steam, and utilizing the specific actual enthalpyas an auxiliary control quantity.
 2. The method of claim 1 wherein theheat exchanger is supplied with the operating mediums of fuel andfeedwater and is equipped with adjusting means for adjusting the flow ofat least one of the operating mediums to the heat exchanger, comprisingcontinuously measuring the actual pressure (pi) and actual temperature(Ti) of the slightly superheated steam in the heat exchanger, supplyingthe measured actual values of pressure (pi) and temperature (Ti) to acomputation circuit for determining the control deviation (xwi) of thespecific actual enthalpy (ii) from the reference value of the enthalpy(io), and supplying the control deviation (xwi) of the specific enthalpyas a correction quantity for the input quantity of the adjusting means.3. The method of claim 2 wherein the adjusting means includes acontroller connected to a positioning device for adjusting the flow ofone of the operating mediums to the heat exchanger, comprising supplyingthe control deviation (xwi) of the specific enthalpy as a correctionquantity for the input quantity (e) of the controller.
 4. The method ofclaim 2 comprising supplying respective predetermined reference valuesof pressure (po) and temperature (To) for the slightly superheated steamto the computation circuit, and determining the control deviation (xwi)of the specific enthalpy from the measured actual values of the pressure(pi) and temperature (Ti) and the predetermined refernce values ofpressure (po) and temperature (To).
 5. The method of claim 4 comprisingdetermining the specific enthalpy (xwi) according to the equation: xwiKp xwp + KT xwT wherein Kp and KT are predetermined values and wherein(xwp) is the control deviation (pi - po) of the pressure and (xwT) isthe control deviation (Ti - To) of the temperature.
 6. The method ofclaim 5 wherein the heat exchanger supplies a load and wherein thevalues of pressure (po) and temperature (To) are functions of the load.7. The method of claim 5 wherein the heat exchanger supplies a load andwherein the values of (Kp) and (KT) are functions of the load.
 8. Themethod of claim 5 wherein the heat exchanger supplies a load and whereinthe values of pressure (po) and temperature (To) and the values of (Kp)and (KT) are all functions of the load.
 9. The method of claim 5 whereinthe heat exchanger is connectable to a turbogenerator for supplying thesame, the turbogenerator having a specified value (Po) of electriccapacity, and wherein the values of pressure (po) and temperature (To)are functions of (Po).
 10. The method of claim 5 wherein the heatexchanger is connectable to a turbogenerator for supplying the same, theturbogenerator having a reference value (Po) of electric capacity, andwherein the values of (Kp) and (KT) are functions of (Po).
 11. Themethod of claim 5 wherein the heat exchanger is connectable to aturbogenerator for supplying the same, the turbogenerator having areference value (Po) of electric capacity, and wherein the values ofpressure (po) and temperature (To) and values of (Kp) and (KT) are allfunctions of (Po). of a i i
 12. The method of claim 2 wherein the heatexchanger operates under the condition that the reference value of thepressure (po) is variable and the reference value of the enthalpy (io)of the slightly superheated steam is constant, and wherein the methodcomprises forming a pressure dependent auxiliary quantity as a functionof the measured actual pressure (pi), said quantity being in the form ofreference temperature (Ts), subtracting the reference temperature (Ts)from the measured actual temperature (Ti) to determine a temperaturedeviation (Ti- Ts), and determining the control deviation (xwi) of thespecific enthalpy as a function of the temperature deviation (Ti- Ts).13. The method of claim 2 wherein the heat exchanger operates under thecondition that the reference value of the pressure (po) is substantiallyconstant and that the reference enthalpy (io) of the slightlysuperheated steam is constant, and wherein the method comprisesdetermining the control deviation (xwi) of the specific enthalpyaccording to the equation: xwi ( Delta i/ Delta T)p (Ti - Ts) whereinthe quantity ( Delta i/ Delta T)p is predetermined as dependent upon theoperating point (io, po, To) of the slightly superheated steam, andwherein the quantity (Ts) is a reference temperature dependent uponpressure and determined from the equation: Ts To + aT , p (pi - po)wherein aT , p is fixed.
 14. The method of claim 2 wherein the heatexchanger operates at a reference enthalpy (io) of the slightlysuperheated steam which varies about a mean enthalpy value (io),comprising: forming a pressure dependent reference temperature (Ts) as afunction of the measured actual pressure (pi), subtracting the referencetemperature (Ts) from the measured actual temperature (Ti) to determinethe temperature deviation (Ti - Ts), determining a control deviationquantity (x*wi) as a function of the temperature deviation (Ti - Ts),and subtracting an intended enthalpy change ( Delta io io - io) from thequantity (x*wi) to form the control deviation (xwi).
 15. The method ofclaim 14 wherein heat exchanger supplies a load and wherein the intendedenthalpy change ( Delta io) is a fuNction of the load.
 16. The method ofclaim 15 wherein the heat exchanger is connectable to a turbogeneratorfor supplying the same, the turbogenerator having a specified value (Po)of electric capacity, and wherein the intended enthalpy change ( Deltaio) is a function of (Po).
 17. The method of claim 14 wherein the heatexchanger is equipped with an injection cooler supplied with intenselyheated steam and a superordinated controller, comprising detecting thetemperature difference (T''i - T''o) of the intensely superheated steamat the injection cooler and forming therewith the deviation quantity(x''w T), and supplying the deviation quantity (x''w T) to thesuperordinated controller to form the quantity ( Delta i''o) forcorrecting the enthalpy deviation ( Delta io).
 18. In a heat exchangersuch as a forced-flow boiler or the like supplied with the operatingmediums of fuel and feedwater and having an evaporator wherein slightlysuperheated steam is generated, the heat exchanger being equipped withan apparatus for controlling the heat exchanger, the apparatuscomprising pressure detection means and temperature detection means forcontinuously measuring the pressure and temperature respectively of theslightly superheated steam at the output of the evaporator, computationmeans connected to said detection means for continuously determining thespecific actual enthalpy of the slightly superheated steam, and meansconnected to said computation means for controlling the heat exchangerin response to said specific actual enthalpy.
 19. The apparatus of claim18 wherein the apparatus controls at least one of the operating mediumssaid last-mentioned means being adjustment means for adjusting the flowof at least one of said mediums to the heat exchanger in response tosaid specific actual enthalpy.
 20. The apparatus of claim 19 wherein theheat exchanger is equipped with a conduit to supply the feedwater to theheat exchanger proper, said adjustment means comprising positioningequipment, said equipment including a valve for controlling the supplyof feedwater to the heat exchanger, and a positioning device connectedto said valve for adjusting the position of said valve.
 21. Theapparatus of claim 19 wherein the heat exchanger is equipped with aconduit to supply the fuel to the heat exchanger proper, said adjustmentmeans comprising positioning equipment said equipment including a valvefor controlling the supply of fuel to the heat exchanger, and apositioning device connected to said valve for adjusting the position ofsaid valve.
 22. The apparatus of claim 19, said pressure detection meanscomprising a pressure transmitter placed at the output of theevaporator, and a pressure transducer connected to said pressuretransmitter for converting said pressure to a corresponding electricalquantity; said temperature detection means comprising a temperaturetransmitter placed at the output of the evaporator, and a temperaturetransducer connected to said temperature transmitter for converting saidtemperature to a corresponding electrical quantity; said computationmeans comprising respective reference transmitters for providingrespective predetermined electrical reference quantities of pressure andtemperature for the slightly superheated steam, and further computationcircuit means connected to said reference transmitters and saidtransducers for forming a control deviation signal corresponding to thedeviation of the specific actual enthalpy from the reference value ofthe enthalpy; and said adjustment means comprising means responsive tothe control deviation signal for adjusting the flow of at least one ofthe operating mediums to the heat exchanger.